Chain length on 8-speed
Chain length seldom needs to be determined with any precision when there’s only one chainring. But if you want a rule, I would make it so the cage is approximately vertical (i.e. one pulley directly below the other) when engaged with a sprocket on the big side of middle, i.e. 4th gear out of eight or nine, 3rd out of seven.
This was first published in the February / March 2013 edition of Cycle magazine.